Transition metal salts comprising an organometallic cation and a non-nucleophilic counteranion have been shown to have utility as photochemically activated initiators for cationic addition polymerization. These photoinitiator salts include (cyclopentadienyl) (arene) iron+ salts of the anions PF6− and SbF6−. Similarly, certain classes of these salts are known to be thermally-activatable curatives for cationic polymerizations.
For many commercial applications, the monomers being polymerized are often multifunctional (i.e., contain more than one polymerizable group per molecule), for example, epoxides, such as diglycidyl ethers of bisphenol A (DGEBA). Mixtures of multifunctional monomers such as epoxides and polyalcohols (polyols) or polyepoxides and polyalcohols can undergo acid-catalyzed polycondensation via a step-growth mechanism. Also included in this description are multireactive monomers—those that comprise two or more classes of reactive groups.
In many applications photoinduced polymerization is impossible, impractical or undesirable. For example, many situations where polymerization reactions occur in a closed environment (i.e., in a mold or in a laminated product) or where polymerizable compositions may contain opacifying pigments, thermally activated initiators are preferred. Thermally-activated initiators, such as known organometallic salts, may be used to initiate polymerization in these cases.
There is a continuing need to be able to modify the rate and temperature of polymerization of energy polymerizable compositions to meet the needs of specific applications.